Polyester nonwoven fabrics and, in particular, meltblown nonwoven fabrics have been used in a variety of filtration and/or barrier-like applications. As an example, polyester nonwoven webs have been used in bag filters and vacuum cleaner filters such as, for example, as described in U.S. Pat. No. 5,080,702 to Bosses, U.S. Pat. No. 5,205,938 to Fiumano et al. and U.S. Pat. No. 5,586,997 to Pall et al. In this regard, paper/meltblown laminates for filtration applications are also known such as, for example, those described in U.S. Pat. No. 5,437,910 to Raabe et al. In addition, polyester nonwoven webs have also been used for filtering biological fluids such as described in U.S. Pat. No. 5,652,050 to Pall et al.
It is generally known in the art that meltblown fiber nonwoven webs often lack the fiber strength and/or tenacity required for certain uses or applications. In this regard, it is known to laminate one or more durable fabrics to the meltblown nonwoven web in order to provide a laminate structure with improved overall characteristics. As an example, U.S. Pat. No. 4,041,203 to Brock et al., U.S. Pat. No. 5,445,110 to Connors and U.S. Pat. No. 5,667,562 to Midkiff describe a durable spunbond/meltblown nonwoven laminate structure which takes advantage of the filtration or barrier properties of the meltblown fabric and the improved strength and durability of the spunbond fabric. The durable spunbond/meltblown/spunbond nonwoven laminate described in Brock et al. is particularly well suited for various uses requiring improved laminate strength and abrasion resistance such as, for example, use as sterilization wrap. While many nonwoven polyester fabrics exhibit excellent strength and durabilty, polyester meltblown nonwovens do not exhibit high strength and durability since the meltblowing process does not adequately draw the fibers so as to significantly promote crystallization of the polymer. Thus, it is likewise known in the art to improve the strength and durability of meltblown polyester materials by laminating a separate durable fabric thereto such as, for example, a spunbond fiber web or other suitable supporting fabric. As a particular example, meltblown polyester nonwoven webs can be laminated with durable fabrics such as those comprising high strength polyester filaments such as, for example, those described in JP Kokai Patent Application No. Hei 7-207566. The polyester filaments have improved strength since they have undergone separate drawing steps which orient the polymer thereby improving the strength and tenacity of both the fibers and the fabric made therefrom. The meltblown fiber web and the drawn fibers may be thermally point bonded to one another. Additionally, it is noted that utilizing one or more support layers can significantly increase the overall cost of the laminate as the need for the supporting material requires additional processing steps to bring the materials together and also a bonding step. Further, formation of both filtration grade materials and high-strength materials often requires significantly increased capital costs as requiring separate and distinct production equipment.
While there exist multilayer laminates that have excellent strength and durability, often the means for permanently bonding the individual layers together can negatively impact the filter efficiency and life. As an example, spunbond and meltblown fiber nonwoven webs are often thermally point-bonded. The bonded areas are highly fused areas which allow little, if any, penetration of the fluid to be filtered. Thus, the bond areas reduce the effective area of the filter and increase pressure drop across the filter media. In addition, use of adhesives and other bonding methods can likewise negatively impact filter efficiency and/or life. Thus, improved abrasion resistance and/or laminate integrity achieved in this manner often comes at the expense of the overall permeability and/or filtration efficiency of the fabric. Consequently, the ability to achieve such improved properties without sacrificing other desired attributes of the filter media material has proven difficult.
As a result, there exists a need for a filter media having improved strength and/or durability. Further, there exists a need for such a filtration media that can be manufactured more efficiently and cost effectively than heretofore. Still further, there exists a need for such filter media that can withstand rigors of converting and use.